A member for transferring heat from a heat generating member to a heat dissipating member in an electric/electronic instrument must be excellent in both heat conductivity and electrical insulation property. A heat conductive sheet produced by blending an inorganic filler excellent in heat conductivity and electrical insulation property is widely used as a member satisfying the requirements mentioned above. Here, examples of an inorganic filler excellent in heat conductivity and electrical insulation property include alumina, boron nitride, silica, and aluminum nitride. In particular, hexagonal boron nitride (h-BN) is especially suitable for use in a heat conductive sheet, because hexagonal boron nitride is excellent in chemical stability in addition to heat conductivity and electrical insulation property, is non-toxic, and is relatively inexpensive. Hexagonal boron nitride has a scaly shape, and is also called scaly boron nitride in the art.
As a heat conductive sheet containing boron nitride, there is proposed a heat conductive sheet produced by dispersing, in a thermosetting resin matrix, secondary particles having isotropic heat conductivity, such as secondary aggregated particles formed by aggregating primary particles of scaly boron nitride or secondary sintered particles obtained by sintering the secondary aggregated particles (see, for example, Patent Documents 1 and 2). Such heat conductive sheet has increased heat conductivity in a thickness direction of the sheet by virtue of the secondary particles having isotropic heat conductivity.
Meanwhile, in recent years, with development of large-current electric/electronic instruments having high pressure resistance, the temperature of heat generated by various semiconductor devices has been increasing. Thus, in order to dissipate the heat of various semiconductor devices efficiently, heat conductivity of a heat conductive sheet is improved by increasing the blending amount of an inorganic filler. However, as the blending amount of an inorganic filler is increased in a heat conductive sheet, defects such as voids and cracks are liable to occur in the heat conductive sheet, resulting in deterioration of the electrical insulation property of the heat conductive sheet. Thus, a pressing step is performed at the time of manufacturing the heat conductive sheet, thereby suppressing occurrence of the defects in the heat conductive sheet. For example, in a power module including a heat conductive sheet, a pressing step is performed at the time of manufacturing a heat conductive sheet in a B-stage state (hereinafter, referred to as “B-stage heat conductive sheet”) before being incorporated into the power module, thereby suppressing occurrence of defects in the B-stage heat conductive sheet. Further, when a B-stage heat conductive sheet is arranged between a lead frame mounted with semiconductor devices and a metal sheet and then, the whole is sealed with a sealing resin by performing transfer molding, thus producing a power module, molding pressure at the time of performing the transfer molding suppresses occurrence of defects in the heat conductive sheet.